1. Field of the Invention
This invention relates to shafts for eddy current probes and, specifically, to a shaft with a prestretched sleeve over an unstretched inner core.
2. Prior Art
It is known to have an eddy current probe for remotely obtaining nondestructive measurements of the integrity of tubes in nuclear steam generators and heat exchangers. The eddy current probe is pushed through a tube or pipe by a flexible shaft to which it is attached on the shaft lead end, the shaft extending from the probe to a data recorder with data cables running along the shaft. As the shaft impels the probe into the pipe, the probe measures the pipe along the pipe length, transmitting probe measurement data through cables along the shaft.
Because the nuclear industry heat exchanger pipes have a tight bend radius, typically less than 2 inches, the probe and shaft must negotiate around such tight bends. Without the capability of negotiating tight-radii bends, it becomes impossible to perform a full examination of the pipes from one pipe end to another, requiring multiple passes through the pipe from different pipe access locations. Completing a full pipe measurement then results in increased inspection time, increased exposure to personnel setting up the measurement equipment, and increased damage to measurement equipment.
Flexible shafts typically have a structure that allows them to bend within a curved pipe and provide for data cables running with the shaft. Such probe shafts accommodate such tight bends mechanically, however, electrical problems remain.
Historically, there has been an inherent problem associated with bobbin type probes commonly referred to as signal drift. Signal drift manifests itself as a meandering back and froth of the eddy current signal from the null or balance point. In addition to making data analysis very difficult, the signal produced could mimic some types of anomalies in the tubing being tested such as dents, bulges, diameter changes, or changes in the wall thickness of the tubing.
The cause of this phenomenon has long been known and attributed to changes in the condition of the coaxial cables. The motion of bending the cables when driving the probe on and off the probe driver take-up reel (drum) as well as over and back around U-bends in the tubing being tested causes such changes. Bending applies stress to the probe shaft and coaxial cables. Stressing the coaxial cables causes changes in the thickness of the dielectric material that insulates the outer shield of the coax from the center conductor of the cables. This results in changes in the capacitive properties of the coax. Change in these properties effects change to the test instrument null or balance point which causes the signal to drift. A similar effect can be noticed by simply moving the strands of cables in proximity to each other.
It is the primary object of this invention to eliminate capacitance variation in eddy current shafts by eliminating changes in cable insulative properties caused by cable stress from bending.